fix: preserve sync RPC empty params shape

[luyua9]

What was wrong?

Follow-up to #3823 / #3825.

#3825 fixed the async persistent-provider path by replacing params or [] with [] if params is None else params, preserving empty tuple/dict params for cache-key consistency.

The sync JSONBaseProvider.encode_rpc_request() path still used params or [], so an empty named-params payload ({}) was serialized as positional params ([]). That changes JSON-RPC request shape before the request is sent.

How was it fixed?

• Update sync JSON-RPC encoding to only default None to [].
• Add a regression test for empty dict vs None on sync encoding.
• Add a bugfix newsfragment.

Validation

• git diff --check
• python -m pytest -q tests/core/caching-utils/test_request_caching.py -k 'sync_encode_rpc_request_preserves_params_shape or async_form_request_preserves_params_shape' could not run in this local checkout because cached_property is not installed in the environment (ModuleNotFoundError).